A piezo-injector of this type is used in connection with the fuel injection process in motor vehicles, in particular in motor vehicles that comprise a common-rail fuel injection system.
A piezo-injector of this type comprises a piezo-actuator and a nozzle needle that is actuated by said piezo-actuator. The piezo-actuator that is embodied in the form of a piezo-stack represents a capacitive load and it can be extended by means of being charged to an electrical voltage that amounts for example to 150V. In the case of a directly driven fuel injection system, this piezo-actuator can be used to transmit the piezo-movement and/or the piezo-force to the nozzle needle by way of reversing the effective direction, for example using a lever.
During the charging phase of the piezo-actuator for the purpose of opening the nozzle needle, an idle stroke is initially overcome and only then is the nozzle needle opened. The term “idle stroke” or “idle travel” describes the distance over which the piezo-actuator must extend to a great extent in a force free manner prior to the force becoming effective on the nozzle needle. It is necessary in dependence upon the system pressure for the drive unit to overcome a defined force in order to raise the nozzle needle from its needle seat. The movement of the piezo-actuator is then transmitted to the movement of the nozzle needle.
The idle stroke has consequently a decisive influence on the point in time at which the nozzle needle rises from its needle seat.
In known injection concepts, it is necessary to place high demands on the components in order to minimize the unavoidable effects of manufacturing tolerances on said components. Irrespective of this, in addition, comprehensive influence analyses and calibration processes are performed, during which the respective concept behavior is ascertained and any effects of manufacturing tolerances are compensated for during the serviceable life of the component.
Furthermore, in the case of known injector control concepts, it is possible to monitor the respective desired injection fuel quantity by means of transmitting electrical feedback signals, which transmission is possible as a result of said nozzle needle being directly coupled to the piezo-actuator, and where necessary to compensate for any changes in the idle stroke.
A disadvantage of the abovementioned option resides in the fact that, for example, when the engine is in the coasting mode in which fuel is not being injected, it is not possible to observe any change in the idle stroke as in this case there is no feedback with regard to the needle position. As a consequence, as the fuel injection process is re-instated, several injection cycles are initially required in order to be able to ensure an acceptably accurate detection of the needle position, which is in turn a prerequisite for compensating any changes that occur in the idle stroke. A temperature model of the piezo-injector can have a supporting effect during this process. However, even in this case, it cannot be sufficiently guaranteed that the accuracy of the fuel injection quantity is ensured.
In the case of in-line injectors, the positions of the components, drive unit and nozzle needle change greatly in particular as a result of thermal expansion. This renders it necessary to continuously correct the fuel injection quantity. A prerequisite of this continuous correction is that the idle stroke of the piezo-injector is ascertained precisely.